As semiconductor devices have been finer and more highly integrated in recent years, there has been an increasing need for silicon wafers of higher quality. This also underscores the need to reduce crystal defects that occur in the course of producing silicon single crystals.
[Defects Contained in Single Crystals and Behavior thereof]
It has been understood that the following three types of crystal defect are commonly included in single crystals and related to deterioration of the performances of a device.    [1] Void defects thought to occur through the agglomeration of vacancies    [2] Oxidation induced stacking faults (OSF)    [3] Dislocation clusters thought to occur through the agglomeration of interstitial silicon
It is known that the way these defects occur varies as follows with the growth conditions.    [i] When the growth rate is high, single crystals have excessive vacancies and therefore prone to the occurrence of only void defects.    [ii] If the growth rate is reduced, ring-like OSF occur around the outer periphery of the crystals, and void defects are present on the inside of the OSF portion.    [iii] If the growth rate is further reduced, the radius of the ring-like OSF decreases, dislocation clusters are produced on the outer side of the ring-like OSF portion, and void defects are present on the inner side of the OSF portion.
If the growth rate is reduced further yet, dislocation clusters are produced throughout the crystal.
It is recognized that the above phenomena occur because a crystal changes from a state of excess vacancy to a state of excess interstitial silicon as the growth rate is lowered, and this change is understood to commence from the outer peripheral of the crystal.
[Defect-Free Crystals (Defect-Free Silicon Single Crystals)]
As mentioned above, as device performances become more sophisticated, there is an increasing need to reduce the crystal defects that occur in the course of producing silicon single crystals. With this in mind, there have been studies into the possibility of producing defect-free crystals (perfect crystals), and a method for producing defect-free silicon single crystals has been proposed in Japanese Laid-Open Patent Application H8-330316 (hereinafter referred to as “Publication 1”).
Publication 1 states that a defect-free region was found where none of the above-mentioned three types of defect is present between the ring-like OSF portion and the region where dislocation clusters occur. This defect-free region is understood to correspond to a transition region from an excess vacancy state to an excess interstitial silicon state, and correspond to a neutral state that does not reach an excess amount at which any of the defects occur.
Publication 1 also states a proposal of a growth method by which this neutral state is attained throughout an entire crystal. With this proposed method, this neutral state can be attained throughout an entire crystal by pulling up the crystal such that the ratio expressed by V/G is kept within a range of 0.20 to 0.22 mm2/° C. min where V is the crystal pull-up rate (mm/min)and G is the average temperature gradient within the crystal in the axial direction between the melting point of silicon and 1300° C. (° C./mm).
If G is constant in the radial direction, then when G=3.0° C./mm, for example, the pull-up rate V should be controlled to 0.63±0.03 mm/min. This is not impossible in an industrial setting. Still, this only refers to the maximum permissible range, and is not actually practical. This is because if G varies, and is not uniform in the radial direction, the permissible range becomes exceedingly small. For example, the permissible range drops to zero once the change in G in the radial direction reaches 10%. This means that slight decreases the uniformity of G make it essentially impossible to produce defect-free crystals (perfect crystals).
Moreover, since G is usually not constant in the radial direction, it is entirely conceivable that the change in G in the radial direction will indeed reach 10%. Because of this, with the method proposed in Publication 1, even if crystals are pulled up at the same pull-up rate, heater output and so forth, defect-free crystals will sometimes be obtained and sometimes not, meaning that the production of defect-free crystals will be extremely unstable.
Furthermore, the following two problems are encountered with the proposal in Publication 1.    [1] G (the average temperature gradient within a crystal in the axial direction) is hard to evaluate and difficult to predict.    [2] G varies during pull-up.
Specifically, factors that cause G to vary during pull-up include changes in the thermal balance resulting from a change in the length of the crystal, changes in the thermal balance resulting from a change in the relative positions of the crucible and heater, and changes in the amount of melt, and it is difficult to ascertain and control these.
Also, whereas the growth rate V is a controllable parameter, the evaluation and prediction of G are very difficult, and change dynamically. Accordingly, a great deal of trial and error are inevitable in the specific implementation of this invention. That is, the relation between the specific settable parameters and the resulting G is unclear, so no specific means is known for achieving this end. Further, even the value of V/G, at which a neutral state is said to be obtained, can vary by two times depending on the research facility, and can even be considered as an uncertain value.
Japanese Laid-Open Patent Application H11-199386 (hereinafter referred to as “Publication 2”) acknowledges the industrial difficulty in producing crystals that will in only this neutral state (method in Publication 1), and proposes a method for producing crystals substantially close to being free of defects, although permitting an OSF portion to remain in just an extremely small region at the crystal center. Publication 2 states that it was believed that the producing conditions under which this state is obtained are determined by V/G, and proposes the following as conditions for getting the entire crystal into this region.    [a] The in-plane average G is less than 3° C./mm, and is less than 1.0° C./mm between Gedge and Gcenter. (Gedge is the average axial temperature gradient on the crystal side face side, and Gcenter is the average axial temperature gradient on the crystal center side. VOSFclose is the pull-up rate at which OSF rings disappear when the pull-up rate is reduced.)    [b] V is controlled to VOSFclose±0.02 mm/min, and the average V is controlled to VOSFclose±0.01 mm/min.    [c] The single crystal pull-up is performed with a magnetic field applied, this magnetic field being a horizontal magnetic field, and the magnetic field strength being 2000 G or greater.
Keeping the difference between Gedge and Gcenter to less than 1.0° C./mm, controlling V to VOSFclose±0.02 mm/min, and controlling the average V to VOSFclose±0.01 mm/min, as in Publication 2, are within the ranges proposed in Publication 1, and what is presented as new information is that it is easier to obtain defect-free crystals when there is a low temperature gradient, with the in-plane average G being less than 3° C./mm, and that the application of a magnetic field is effective.
Japanese Laid-Open Patent Application H11-199387 (hereinafter referred to as “Publication 3”) proposes a method for producing defect-free crystals containing no OSF portion. Publication 3 states that there are two types of neutral region, and noting that there is a defect-free region in which vacancies are predominant and a defect-free region in which interstitial silicon is predominant, the proposal was made of a method for producing defect-free crystals in which interstitial silicon is predominant.
As to the conditions for pulling up defect-free crystals, the in-plane change in G is adjusted so that (Gmax−Gmin)/Gmin will be less than 20%. This is also within the proposed range given in Publication 1, and no specific method is disclosed. The value of G given in Publication 3 is determined by heat transfer analysis (FEMAG), and not only is the absolute value of G not known, it is not even certain whether the distribution trend in the radial direction itself corresponds to actual crystals.
Japanese Laid-Open Patent Application H11-79889 (hereinafter referred to as “Publication 4”) proposes a method for producing crystals so that just the neutral state is produced. This method involves flating of the shape at the solid-liquid interface, and it is proposed that the pulling be such that the height of the solid-liquid interface will be no more than ±5 mm with respect to the average value. In this case, G is uniform, and Gedge and Gcenter can be kept under 0.5° C./mm. Applying a magnetic field is an effect way to obtain a flat solid-liquid interface shape such as this, and a horizontal magnetic field of 2000 Gauss or greater is said to be best.
The new finding of this proposal is that the shape of the solid-liquid interface is identified as a factor. The given G value, however, is found by heat transfer analysis (FEMAG), just as in Publication 3. However, just because the solid-liquid interface is flat does not automatically mean that G will be uniform, so not only is the absolute value of G not known, it is not even certain whether the distribution trend in the radial direction itself corresponds to actual crystals.
As discussed above, with prior proposals, crystals free of defects could be obtained if the growth rate V and the axial temperature gradient G near the solid-liquid interface were appropriately controlled. However, as described above, in addition to the fact that G is dynamic, changing from moment to moment while a crystal is being pulled up, it is also extremely difficult to evaluate or predict this value accurately. Actually, even the value of V/G, at which a neutral state is said to be obtained, can vary by two times depending on the research facility, and can even be considered an uncertain value.
Thus, whereas the growth rate V is a controllable parameter, the evaluation and prediction of G are very difficult, and change dynamically. Therefore, a great deal of trial and error were inevitable in the specific implementation of the inventions according to the above-mentioned conventional technology.
Also, the relation between the specific settable parameters and the resulting G is uncertain, so no specific means is known for determining the proper G in all publications related to the aforementioned prior technology.